Since becoming Auckland District Commander and developing an appreciation for wine — the two were related — Finbar McGrail hadn’t been sleeping as well as he used to.

His late, grimly Presbyterian mother was fond of saying ‘the sleep of the righteous is sweet’, a paraphrase of Proverbs 3:24. While McGrail was confident that promotion hadn’t disabled his moral compass, he had to acknowledge that he’d gone from having an occasional glass of wine to not needing an occasion to open a bottle, and from dutifully saying his prayers to no longer bothering to touch base with God before calling it a night.

First the formalities were dispensed with: the kneeling beside the bed, head bowed (because although heaven is commonly thought to be up there somewhere, presumably above airliners’ cruising altitude, believers know that God is everywhere, even under one’s bed), hands clasped, eyes shut, the constipated expression of rapture tempered by obeisance. Once he started saying his prayers after, rather than before, getting into bed, it was a short step to mouthing the words, as opposed to saying them out loud, and an even shorter step to thinking them. And once the process was internalised, it was difficult not to get distracted or deflected. It was almost as if his mind had a mind of its own.

Eventually McGrail gave up the struggle and fell into the habit of thinking about work for however long it took for his wife’s current book to make her eyelids droop. When she said good-night and turned off the bedside light, he would roll onto his side and go to sleep, albeit not without a twinge of guilt, like someone who has let another day go by without ringing his aged parents.

As often as not these days, McGrail would wake up in the early hours. Rather than wait for the fog of sleep to roll back in or engage in that erratic, tangential mental activity that seems productive, even inspired, at 3 am but turns out to be inconsequential at best when retrieved in the morning, he’d slip out of bed. After making himself a cup of cocoa, he’d go into his study to chip away at his email backlog, which was seldom less than a hundred messages.

Before he went back to bed, McGrail would look at a photo that he still kept in his bottom drawer even though his children had left home. It was a head-and-shoulders shot of a teenage girl trying to put on an exasperated ‘Do I really have to do this?’ expression but unable to keep the smile off her face. McGrail knew a lot about this girl, whose name was Polly Stenson. For instance, he knew that she’d had her braces removed a fortnight before the photo was taken. The orthodontist had met the challenge he’d been set two years earlier: to have Polly’s teeth straight and unencumbered by her seventeenth birthday.

The date print-out, in orange lettering, on the bottom right-hand corner of the photo said 15. 8. 87. It was taken on the last afternoon of Polly Stenson’s short life.

McGrail had been in New Zealand a fortnight, having left Northern Ireland even though — in fact because — he was a rising star in the Royal Ulster Constabulary. The Stenson murder was the first case for Auckland Central’s new Detective Inspector, of whom much was expected.

Polly was murdered at an election-night party held at the spectacular Remuera home of merchant banker Tim Barton and his wife Nicky. Barton had called it a ‘Win-Win’ party because, as far as people like him were concerned, it made no difference who won the election. That was understandable: it seemed to McGrail that the only real economic disagreement between the two major parties was over which of them was the more laissez-faire.

McGrail was taken aback by the ostentatious displays of wealth and unashamed extravagance he encountered during the investigation. He and his wife had decided to immigrate to New Zealand after extensive research and the process of elimination led them to the conclusion that their people — the Protestants of Ulster — had more in common with New Zealanders than any other nationality.

McGrail was under the impression that New Zealanders were stoic, understated, laconic to the point of taciturnity, suspicious of self-promotion and public display, inclined to pessimism and quick to say ‘I told you so’ when their gloomy prognoses were borne out. The glaring difference was that New Zealanders didn’t seem to take religion anywhere near seriously enough to kill or maim their neighbours over denominational differences. (That was an aspect of life in Northern Ireland that McGrail was keen to put behind him: he had thought about emigrating for years, but the tipping point was finding out that his name was on a Provisional IRA hit-list. While hit-lists were a dime a dozen in Belfast — there were pub darts teams who had them — the Provos had already put a black line through some of the names on theirs.) If McGrail had wanted nouveau-riche vulgarity, he would have gone to America, or even Australia. Thankfully it didn’t last. After the Black Tuesday sharemarket crash later that year, New Zealanders reverted to type. For a while, anyway.

The investigation was a nightmare. Over the course of the evening at least three hundred people had passed through the Barton mansion, but that was a woolly estimate since there were no formal invitations or guest list: Barton had just put the word out to his friends who passed it on to their various overlapping social circles. Barton’s twenty-one-year-old son Johnny and teenage daughter Lucy had hosted their own sub-parties.

There was no proper security and therefore no one with a sober recollection of comings and goings or who was likely to notice odd or jarring behaviour. Security, such as it was, was provided by Johnny’s rugby team who were given the narrow brief of repelling any uncouth elements that might try to crash the party. Predictably, most of the rugby players got drunker and did so faster than the other attendees.

Polly and Lucy went to the same girls’ private school, although Polly was a bit of an outsider. Her father was middle management; she was a scholarship girl, bright and athletic. Although her friends had done their best to corrupt her, Polly stuck to her unfashionable principles relating to booze, drugs and what was a seemly level of sexual activity for a girl her age.

By midnight, most of the girls and their dates — Polly was one of the few in the group who didn’t have a boyfriend — were too tipsy or distracted to look out for their friends. But then why would they? If you weren’t safe in that grand house in one of Auckland’s most prestigious streets surrounded by hundreds of people, including MPs from both major parties and an array of Rich Listers and movers and shakers, where on earth would you be?

The Barton place was on three levels. The ground floor was the living and entertainment area. Downstairs was the kids’ domain: the only adults who ventured below were the cleaning ladies. Upstairs was the parents’ quarters, complete with his-and-hers studies, library, gymnasium and sauna. It was well understood that upstairs was a child-free no-go zone.

The adults had congregated on the ground floor. The younger generation had split into groups: despite the time of year the rugby players yahooed around the pool; Lucy and friends mainly stayed downstairs; the little band of dope smokers had made their furtive way to the tennis court.

Polly had arranged to sleep over at another friend’s house; a cab was booked for 2 am. Around 11.30 pm, without saying where she was going or why, she went upstairs. She had a brief exchange with Tim Barton, telling him she felt like getting some fresh air. He later said she seemed fine: she’d obviously had a few drinks but wasn’t drunk, disoriented or looking for trouble.

Outside she bumped into a friend’s boyfriend who’d been out on the tennis court where the joints were circulating. She told him she was taking time out from the tiresome boy-girl interaction downstairs. He advised her to get stoned, knowing there was zero chance of that happening. That was the last anyone saw of her.